Auto merge of #17821 - bzbarsky:pseudo-prop-restrictions, r=emilio

Implement restrictions on which properties apply to which pseudo-elements

::first-line, ::first-letter, and ::placeholder don't allow all properties to apply to them.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1382786

<!-- Either: -->
- [ ] There are tests for these changes OR
- [X] These changes do not require tests because they're Gecko-only for the moment, and Gecko has tests.

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17821)
<!-- Reviewable:end -->

components/style/gecko/pseudo_element.rs

@@ -10,6 +10,8 @@
 
 use cssparser::{ToCss, serialize_identifier};
 use gecko_bindings::structs::{self, CSSPseudoElementType};
+use properties::{PropertyFlags, APPLIES_TO_FIRST_LETTER, APPLIES_TO_FIRST_LINE};
+use properties::APPLIES_TO_PLACEHOLDER;
 use selector_parser::{NonTSPseudoClass, PseudoElementCascadeType, SelectorImpl};
 use std::fmt;
 use string_cache::Atom;
@@ -119,4 +121,15 @@ impl PseudoElement {
             _ => self.clone(),
         }
     }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        match *self {
+            PseudoElement::FirstLetter => Some(APPLIES_TO_FIRST_LETTER),
+            PseudoElement::FirstLine => Some(APPLIES_TO_FIRST_LINE),
+            PseudoElement::Placeholder => Some(APPLIES_TO_PLACEHOLDER),
+            _ => None,
+        }
+    }
 }